Techniques have long been known for depositing substances, such as layers of semiconductor material, using a plasma that is formed into a jet. For example, U.S. Pat. Nos. 4,471,003 and 4,487,162 disclose arc jet plasma deposition equipments which utilize a plasma for deposition of semiconductors and other materials. Ions and electrons are obtained by injecting an appropriate compound, such as a silicon compound, into an arc region, and a jet (or beam) is formed by utilizing magnetic fields to accelerate and focus the plasma. Recently, equipment of this type has been used to deposit synthetic diamond. Superior physical and chemical properties make diamond desirable for many mechanical, thermal, optical and electronic applications, and the ability to deposit synthetic diamond by plasma jet deposition holds great promise, particularly if plasma jet techniques can be improved for this and other purposes. A plasma of a hydrocarbon and hydrogen can be obtained using electrical arcing, and the resultant plasma focused and accelerated toward a substrate, using focusing and accelerating magnets, so that polycrystalline diamond film is deposited on the substrate. Reference can be made, for example, to U.S. Pat. No. 5,204,144, assigned to the same assignee as the present Application, for description of an example of a type of plasma jet deposition that can be utilized to deposit synthetic diamond on a substrate.
In various commercial applications it is desirable to have relatively large size diamond films. In plasma jet deposition techniques there are various factors which limit the practical size of the deposition area that is active on a substrate at a particular moment. For example, when an arc is employed to generate the heated gas mixture in an arc jet plasma deposition system, the diameter of the beam can be limited by a number of factors. Since the cross-section of the plasma beam is generally limited in practical applications, the area on which it is desired to deposit a diamond film may be larger than the deposition beam. This means that it may be desirable to move the beam and the target substrate with respect to each other during the deposition process. This has been achieved by spinning the substrate during deposition, which helps to promote temperature uniformity over the substrate, as well as to attain larger area substrate coverage (see e.g. the referenced U.S. Pat. No. 5,204,144)
In plasma jet deposition of the type described, it is typically necessary to cool the substrate (or mandrel) upon which the diamond is being deposited, to prevent the hot plasma from overheating the deposition surface, and to provide an optimum deposition temperature for the particular product characteristics desired. A coolant can be circulated in the mandrel to provide cooling. In a rotating mandrel type of deposition equipment, as described in U.S. Pat. No. 5,204,144, cooling fluid can be circulated through a rotating union. FIG. 1 illustrates a prior art type of configuration wherein a rotatable mandrel 110, coupled with a rotary union 115, is rotated, such as by a belt or gear drive (not shown), and coolant fluid can be circulated through the mandrel, in the direction indicated by the arrows, or in the opposite direction. A disc-shaped spacer 150 and substrate 170 are shown as being mounted on the rotating mandrel.
Although the described type of cooling and spinning of the substrate can be effective in obtaining a suitable temperature that is generally uniform azimuthally, temperature at the deposition surface of a disc-shaped substrate can vary substantially in the radial direction, due to factors such as variation in heating by the beam as a function of its cross section.
It is among the objects of the present invention to provide improved temperature control, such as to achieve temperature uniformity, in a plasma deposition apparatus, such as the type used for deposition of synthetic diamond. It is also among the objects of the present invention to provide temperature control in a deposition system in which a substance, such as synthetic diamond, is being deposited on a rotating mandrel or substrate.